All high voltage coils use an outer conductive ground electrode to make certain the outer surface of the high voltage coils is at ground potential. As illustrated in FIG. 1, the ground wall insulation 10 of high voltage stator coils is generally comprised of multiple layers of glass backed mica paper tape 14. The last outer layer is a conductive tape 12 that is loaded with carbon or graphite to control and set its conductivity. During the impregnation and heat cure of the stator coil, shrinkage occurs in the conductive tape 12, which causes the tape to conform to the surface below it.
Currently, the layer of conductive tape 12 is placed on the insulated coils surface in a half lap layer manner (described herein) and applied over the last layer of mica paper tape 14 that is also placed on the coil surface in a half lap layer. This method, while somewhat successful, causes protrusions of either the conductive tape 12 or the mica layer 14.
The smoothness of the conductive tape 12 layer has a big influence on the electrical properties of the other ground electrode. Protrusions 16 of either the conductive tape 12 or the mica tape 14 below it can produce regions of locally enhanced electrical stress. The protrusions 16 created by the crevice created by the half lap interface of the glass backed mica paper tape 14 results in a very pronounced local electrical stress point 16, as illustrated in FIG. 1.
In addition, increased local electrical stress can cause locally produced partial discharges, which can lead to damage of the coil's outer ground electrode. Calculations can estimate the magnitude of the increased electrical stress at the tip of the conductive tape protrusion. The following calculation may be used to estimate the magnitude of this effect:η=(2d)/[rln(r+4d)/r]]where η is the stress enhancement factor; r is the radius of the conductive point and d is the point to plane distance. The stress enhancement is the amount by which the electric field at the tip is increased because of the protrusion of the conductive tape into the crevice of the glass backed mica paper tape.
What is needed is a method that wraps the outer conductive ground electrode with conductive tape 12 such that the conductive tape 12 and the surface to which it is applied, are as smooth as possible, i.e. having minimal protrusions, thereby reducing the enhanced electrical stress on the ground electrode.
Other difficulties with the prior art also exist, some of which will be apparent upon further reading.